The invention relates to the preparation of coatings on wires or fiber-like materials, such as glass fibers, for example for obtaining insulations on electrically conducting wires, in particular for obtaining solderable enameled wires or for preparing coated glass fibers.
The use of polyurethane coatings for electrical insulation of copper winding wires is known. This type of wire enamel is used extensively for the preparation of easily tin-coatable and thus solderable enameled wires for the electrical industry.
The preparation of polyurethanes from hydroxyl-containing polymers and polyisocyanates is generally known from DE-A-728 981. DE-A-756 058 describes the preparation of coatings, also including those on wires, by leading the wires through a solvent-containing bath of an enamel based on hydroxyl-containing polymers and polyisocyanates. In practice, the baths and the process proved to be unsuitable, since only coatings having different layer thicknesses could be obtained. For this reason, blocked polyisocyanates were developed which were said not to have this disadvantage (O. Bayer: Neuere Entwicklungen des Diisocyanat-Polyadditions-Verfahrens (Recent Developments in Diisocyanate Polyaddition Processes)/FATIPEC 1957, p. 14 and 15 and Houben-Weyl, 1962, vol. 14/2, p. 78 and BAYER-Produkte fur die Elektroisolierung (BAYER Products for Electrical Insulation), item 1.1/2 to 1.1/3 , version 1987).
In practice, enameled wires are still prepared by using combinations of hydroxyl-containing polymers, such as, for example, polyester resins together with phenol-blocked polyisocyanates in a joint solution. Blocking of the polyisocyanate component affords protection against premature gelling of the coating and results in a sufficient shelf life of these single-component coatings. A typical example of phenol-blocked polyisocyanates is the adduct of trimethylolpropane with diisocyanatotoluene and phenol (Desmodur AP.RTM.), such as is described in Houben-Weyl, 1962, vol. 14/2, p. 67. Typical solvents for these coatings are technical grade cresols in a mixture with aromatic hydrocarbons. The coatings usually have a solids content of about 25-35%.
The enameled wires are usually prepared by several continuous applications of the wire enamel onto the metallic wire by means of felt or die wiping. After each application, the coating is cured at temperatures between 300.degree. and 500.degree. C. The phenolic blocking agent is eliminated during the crosslinking process and leaks out of the coating film. Temperature and residence time in the wire-enameling oven depend on the diameter of the wire and the structural design of the oven.
The use of phenolic solvents and blocking agents has disadvantages. The formulation of completely cresol-free, environmentally friendly polyurethane wire enamels is not possible, since even in a cresol-free solution the phenolic blocking agent is released during curing. During the curing process, additional energy has to be expended for eliminating the blocking agent.
The high temperature of 300.degree. to 500.degree. C. in the wire-enameling oven, which is necessary for film formation, can lead to pyrolytic degradation reactions already during the baking process with the elimination of isocyanates and amines.
Various experiments to prepare cresol-free wire enamels based on blocking agents other than phenol/cresol are described in DE-A-2 404 740, 2 545 912, 2 626 175, 2 632 037 and 2 840 352. These processes have technological disadvantages and have not been successful.
If alcoholic blocking agents are used, such as are described, for example, in DE-A-26 26 175, their deblocking temperature is increased even further and the risk of thermal damage during curing becomes greater.
DE-A-25 45 912 describes the use of lactam-blocked polyisocyanates. The caprolactam which is eliminated during the baking process condenses in the cooler waste air ducts of the wire-enameling oven and has to be removed from there by mechanical means. The obtainable coating rates are reduced compared with those of the phenol-blocked isocyanates.
The Applicant's DE-A-20 22 802 describes a process for the preparation of a plastic cover for metallic wires. To this end, a film bubble of thermosetting resin is extruded onto a hollow mandrel, the electric conductor being taken off at a higher rate. In the case of resins present as a liquid, they are completely or partly prereacted in order to obtain the necessary viscosity at the die head. Crosslinkable plastics of this type mentioned also include reaction products of polyisocyanates with hydroxy polyesters. However, the high viscosity causes an undesirable stretching of the wire and even wire cracks as a result of high tensile forces, especially in the case of thin wires.